Conventionally, there is a common structure of a crystal-system silicon (Si) solar cell, which uses an anti-reflective film that is formed on a photoelectric conversion unit having a pn junction formed therein, a comb-shaped front-surface electrode that is formed on a light-receiving-surface side of the photoelectric conversion unit, and a back-surface electrode that is formed on the entire back surface of the photoelectric conversion unit. Because irradiated light is blocked by the front-surface electrode on the light-receiving-surface side, a region covered by the front-surface electrode does not contribute to electric power generation. That is, a so-called shadow loss occurs. The percentage of this shadow loss in the total surface is a little less than 10%.
It is possible to reduce the shadow loss by decreasing the electrode area. However, as the cross-sectional area of the electrode is decreased, the resistance of the electrode is increased, thereby increasing the resistance loss in the front-surface electrode. Because the increase in the resistance loss causes reduction in fill factor (FF), conversion efficiency cannot be increased by simply decreasing the electrode area. When the electrode area is decreased, it is a necessary procedure to reduce the resistance loss, such as by increasing the thickness of the electrode or decreasing the resistivity of an electrode material itself by the amount of the decrease in the electrode area.
As one method to solve the trade-off requirements as described above, a solar cell having a structure in which a front-surface electrode (or a diffusion layer) is arranged on a back surface through a through hole has been studied. The solar cell is referred to as “metal wrap through (MWT) cell” (or “emitter wrap through (EWT) cell” in the case of the diffusion layer) (see, for example, Patent Literatures 1 and 2).
The solar cell having the conventional structure described above has a problem in that when the area of the front-surface electrode is decreased, the resistance loss is increased. However, in the solar cell having the structure as described immediately above, it is possible to use a method in which a bus electrode (all the front-surface electrodes in the EWT cell) is arranged on the back surface, where constraints on the electrode area are eased, to reduce the shadow loss, and also the electrode area is increased to decrease the series resistance. In the solar cell utilizing the through hole as described above, a current collected on the light-receiving-surface side passes through the through hole. Therefore, the number of the through holes and the resistance in the through hole affect characteristics of the solar cell.
For example, a case where one through hole is formed and a case where four through holes are formed are considered in a solar cell having a light-receiving surface area S. Where a photocurrent density is represented as J, and a resistance of the through hole is represented as R, then a resistance loss Ploss1 of the through hole in the case where one through hole is formed is expressed by the following equation (1). On the other hand, a resistance loss Ploss4 of the through hole in the case where four through holes are formed is expressed by the following equation (2). As understood from the equations (1) and (2), as the number of the through holes is larger, the resistance loss can be decreased to a larger extent.Ploss1=J2S2R  (1)Ploss4=(J×¼S)2×R×4=¼J2S2R  (2)
The resistance of the through hole depends on the diameter of the through hole. A current collected on the light-receiving-surface side flows through metal filled in the through hole in the case of the MWT cell, or flows through the diffusion layer on the side of the through hole in the case of the EWT cell. A volume V of a cylinder with a radius r and a height d is expressed by the following equation (3). Further, a lateral area A of the cylinder with the radius r and the height d is expressed by the following equation (4). Therefore, as the diameter of the through hole is larger, the resistance of the through hole can be lower. That is, a larger diameter of the through hole, and a larger number of the through holes are thought to be preferable in order to achieve high photoelectric conversion efficiency in the MWT cell and the EWT cell.V=πr2d  (3)A=2πr  (4)
Next, a manufacturing process of the MWT cell is explained. There is not any significant difference in the manufacturing process between the EWT cell and the MWT cell, except the position of an electrode. A p-type Si substrate (hereinafter, “substrate”) is assumed to be used in this process. However, even when an n-type Si substrate is employed, the same cell can also be produced by changing a diffusion material to an appropriate diffusion material.
First, a through hole is formed on the p-type Si substrate (hereinafter, also “substrate”) by laser. Next, minute recesses and projections referred to as “texture” are formed on a surface of the substrate. The texture reduces the surface reflectivity of a solar cell, and high conversion efficiency can be obtained.
The substrate is then heated in a gaseous atmosphere of phosphorus oxychloride (POCl3), thereby forming an n-type impurity diffusion layer on the surface of the substrate to form a semiconductor pn junction. Next, a plasma-enhanced chemical vapor deposition (PECVD) method is used to form a silicon nitride (SiN) film (a PECVD-SiN film), for example, on the light-receiving-surface side of the substrate as an anti-reflective film.
Vapor phase diffusion is performed using POCl3 to form the n-type impurity diffusion layer, and the SiN film formed by the PECVD method (the PECVD-SiN film) is used as the anti-reflective film. However, a spin on dopant (SOD) can also be used to form the n-type impurity diffusion layer. Further, when the substrate to be used is a single-crystal Si substrate, an alternative is to use a silicon thermally-oxidized film (SiO2) as the anti-reflective film. Furthermore, when phosphorus (P)-doped titanium dioxide (TiO2) is used as the SOD, a process of forming the anti-reflective film simultaneously with forming the n-type impurity diffusion layer is also possible.
Thereafter, an electrode is printed and fired on the front surface and back surface of the substrate including the inside of the through hole, and after undergoing isolation by laser, the MWT cell is completed.
As described above, there are several conceivable processes according to the methods for forming the diffusion layer and for forming the anti-reflective film. However, whether the diffusion layer and the anti-reflective film are present or absent in the through hole significantly affects electrode formation, and therefore caution is necessary. In order to form a front-surface electrode of a solar cell, it is common to adopt a so-called fire-through method in which a conductive paste is printed and fired, thereby breaking an anti-reflective film to come into contact with a diffusion layer under the anti-reflective film.
For example, in a case where only the diffusion layer is present in the through hole, a problem arises in that when a fire-through paste is used to fill the through hole, the paste breaks the diffusion layer within the through hole, and forms a leak path, and therefore the paste does not provide desired properties. In this context, Non Patent Literature 1 describes that it is necessary to use a different paste for a back-surface n-type electrode from that used for a front-surface n-type electrode. Further, Patent Literature 3, in which a through hole is formed after forming a pn junction that is a heterojunction, adopts a procedure to form an insulating film on the side of the through hole before filling a conductive material in the through hole.